Electrochemical cell provided with ion exchange membranes and bipolar metal plates

ABSTRACT

A membrane electrochemical cell, in particular a fuel cell, of an improved type comprising a multiplicity of cell elements, each element made up of bipolar plates, current collectors, electrodes and membranes, wherein the function of electric current transmission through the cell elements, the release of heat towards the outside environment, the distribution of electric current to the electrodes and membranes, the removal of heat from the electrodes and membranes and the distribution of the reactants and products are performed by distinct components, in particular bipolar plates for the first two and porous electroconductive collectors for the others. The bipolar plates may have flat surfaces without grooves and are preferably manufactured with aluminum, titanium or alloys thereof, through cheap mass productions techniques; the bipolar plates are used together with collectors provided with deformability, residual resiliency and high porosity. Said collectors advantageously act also as distributors of the gaseous reactants and of the products.

This application is a continuation of U.S. patent application Ser. No.227,006 filed Apr. 13, 1994, now U.S. Pat. No. 5,482,792.

DESCRIPTION OF THE PRIOR ART

Fuel cells fed with reactants containing hydrogen and oxygen in theanodic (negative polarity) and cathodic (positive polarity) compartmentsrespectively, are apparatuses characterized in that they produceelectric current with energy conversion efficiency referred to theheating value of fuel double or even triple with respect to thosetypical of an internal combustion engine. Further some fuel cells mayalso operate at relatively low temperatures, indicatively in the rangeof 50°-200° C., which make them extremely useful for intermittentoperations such as those typical of both small on-site generation ofelectric energy (for example as required by mechanical workshops) andon-board power generation for transportation means. These applicationsare also favoured by the property of the fuel cells of being absolutelynoiseless, apart from the minor noise connected to the operation ofauxiliary apparatuses, such as fans and pumps for the cooling circuit.Among the various low-temperature fuel cells, particularly attractivefor the above mentioned purposes is the type based on the use of ionexchange membranes, in particular perfluorinated sulphonic membranes.The use of ion exchange membranes, which substitute the conventionalliquid electrolytes, permits the construction of very simple fuel cellsdue to the absence of the circulation, and make-up systems which arenecessary with liquid electrolytes, as well as problems of corrosioncaused by the electrolytes themselves. Said absence of electrolyteresults in a wider choice of materials possibly lighter and moreeconomical. The use of ion exchange membranes, which must be actuallyconsidered as solid electrolytes, poses the problem of the nature of theinterface with the porous electrodes fed with hydrogen and oxygen. Inthe case of the liquid electrolytes, due to the capillary forces, theypenetrate into the pores of the porous electrodes thus forming ameniscus wherein the triple contact between liquid, gas and catalyst ofthe electrode occurs, as it is required for causing the high-speedconsumption of hydrogen and oxygen respectively.

In the case of ion exchange membranes, the contact between the membranesthemselves and the porous electrodes is necessarily influenced by thefact that the two components are solid substances and therefore the areaof triple contact results are limited to the areas of real physicalcontact. Therefore the capillarity phenomena which contribute in such adeterminant way with the liquid electrolytes are not possible. As aconsequence the consumption speed of hydrogen and oxygen is rathersmall. This problem is overcome by heat-pressing the porous electrodesmade of electrocatalytic particles onto the membranes, as described inU.S. Pat. No. 3,134,697. Further improvements have been obtained byadding electroconductive particles, polymeric binders and in particularmaterials capable of favouring the migration of protons as claimed inU.S. Pat. No. 4,876,115.

However, notwithstanding these improvements and implementations, the ionexchange membrane fuel cells have not yet achieved industrial success.One of the reasons for this difficulty resides in the fact that thedesigns of the membrane fuel cells known in the art have not given sofar a satisfactory reply to the problems of safety and fabricationcosts, bound to the types of materials used for the construction, aswell as to the need for mass production and assembling simplicity. Thissituation is due to the fact that the design of the membrane fuel cellmust solve an objectively complex technical problem, that is providingthe anodes at the same time with a homogeneous distribution of bothelectric current and reactants, a complete contact with the membrane andan effective withdrawal of the heat produced by the inefficiencies ofthe system (overvoltages, ohmic drops). The design of the fuel cell ofthe prior art is usually based on the fact that the electrodes mustconstitute a unitary structure with the membrane, obtained as aforesaidby heat-pressing the various components. This unitary structureintrinsically ensures the best continuous contact between the membraneand the electrodes. On these bases, the design of the bipolar plates hasbeen finalized to perform the other tasks of gas and electric currentdistribution and heat withdrawal. The best preferred geometry a bipolarplate provided with grooves, in particular with the grooves of one sideoriented at 90° with respect to the grooves of the other side, asdescribed in U.S. Pat. No. 4,175,165. More particularly, the cathode(positive) compartment where water is formed and the presence of liquidcondensate is more likely to occur, is characterized by the groovesbeing kept in the vertical direction to allow for the best draining. Inthe fuel cell made by a multiplicity of cell elements, each of said cellelements comprises a unitary electrodes-membrane structure rigidlypressed between the two sides of the two adjacent bipolar plates. Inparticular, as the grooves are crossed at 90° the areas with aremarkable contact pressure are those areas where the grooves aresuperimposed and more particularly they are formed by a matrixconsisting of squares having a side equal to the width of the crest ofthe grooves and a pitch equal to the width of the "valleys" of thegrooves. As a consequence the distribution of current and the withdrawalof heat, certainly localized in the areas of greater contact pressure,may be made sufficiently homogeneous only by using very thin grooves andincreasing as much as possible the transversal electrical and thermalconductivity of the electrodes. Therefore, the production costs of thebipolar plates are rather high in consideration of the need tomechanically work the surfaces in an accurate way to obtain the groovesand to ensure the necessary planarity required by a substantially rigidsystem wherein the only element at least partially provided withresiliency is the electrodes/membrane structure. The type of requiredmachining, scarcely compatible with mass production, strongly limits thedimensions of the bipolar plates to values capable only of permittingthe production of small-size electric power systems, such as arenecessary for electric transportation, but certainly too small for otherimportant applications which foresee the on-site stationary generationof electric power, such as required for local electric generators formechanical workshops. The need to limit the costs due to the machininghas forced the choice towards materials capable of being molded orextruded, in particular mixtures of graphite and polymeric binders asclearly described in the aforementioned U.S. Pat. No. 4,175,165.

As the bipolar plates must exhibit a sufficient electrical and thermalconductivity, the content of polymeric binder mixed to the graphite hasto be maintained to a minimum which however must be able to assure thenecessary moldability. As a consequence the toughness of the bipolarplate is not too high, certainly not to be compared with that typical ofmetallic materials. Further, a permeability to gases, even if minimum,cannot be excluded. Therefore, obvious objections as to the intrinsicsafety of fuel cells equipped with graphite bipolar plates ariserelating to the resistance to mechanical shocks and the possible releaseof hydrogen, in particular when operating under pressure. On the otherhand, the metals which are usually considered, that is titanium,niobium, tantalum (known as valve metals, that is capable of formingwith time a protective oxide which is electrically insulating),stainless steels and superalloys, such as the various types ofHastelloy® grades are characterized by high cost, high specificdensities and limited thermal and electrical conductivity. Further, atleast the valve metals must be provided with an electroconductivecoating capable of maintaining a low electrical resistivity, said needfurther increasing the already high costs. It is further possible thatthe design with grooves may bring to anomalous operation, as thedistribution of gas takes place only longitudinally along the grooves,without any appreciable transversal mixing.

As regards the electrodes, the need for a high electrical and thermaltransversal conductivity reduces the selection to a few types and theuse of unitary electrodes/membrane structures involves a furtherproduction step of heat-pressing. This step is doubly expensive in termsof manpower and necessary equipment, such as high-power presses, with acontrolled temperature of the plates and with quite strict planarityrequirements.

A constructive modification disclosed by U.S. Pat. No. 4,224,121comprises the addition of one or more metal meshes between the groovedbipolar plate and the electrodes/membrane unitary structure. Thisarrangement may improve the electric current distribution if at leastthe mesh in contact with the surface of the electrodes has a fine meshsize, even if this does not accomplish the final purpose of a completehomogeneity in the distribution also at a microscale level. In fact, theprivileged areas are those subjected to a higher contact pressurecorresponding to the intersections of the grooves. The addition of apackage comprising a certain number of meshes, furthermore, provides thesystem with a certain resiliency and thus the planarity of the bipolarplates is a less strict requirement.

A bipolar plate design which avoids the complications of the mechanicalworking required for the grooves foresees the use of undulated sheets,optionally perforated, used for electrical contact between the surfacesof the electrodes and those of the planar bipolar plates, as describedin DE 4120359. The undulated sheets may be welded to the bipolar platesor to the surface of the electrodes or to both. In a simpler and lessexpensive embodiment, the undulated sheets are simply pressed betweenthe bipolar plates and the unitary electrodes/membranes structures. Inthis last case the two sheets on the sides of each singleelectrodes/membrane structure must necessarily be positioned crossingthe respective undulations and the areas with a substantial contactpressure are those where the undulations are superimposed. The devicesincorporating the above mentioned undulated sheets are substantiallyaffected by the same shortcomings as discussed for the groovesconcerning the current and gas distribution, and more severeshortcomings as regards the heat removal considering the reducedthickness of the sheets necessary to ensure for a certain resiliency. Itis also evident that the use of undulated sheets requires thatelectrodes and membrane form unitary structures which may be obtained asabove illustrated by heat pressing.

A further construction solution described by the prior art foresees theuse of porous sheets of sinterized metal, directed to act at the sametime as current and gas distributors. In this case the cell element tobe formed by the unitary electrodes/membrane structure pressed betweentwo sheets of sinterized metal, in turn is pressed between the twoplanar bipolar plates, as described in DE 4027655.C.1.

In an alternative embodiment, the unitary structure is formed by amembrane and one electrode only; the second electrode is applied as anelectrocatalytic coating onto the surface of a sinterized metal sheet.The cell element is therefore formed by the electrode/membrane unitarystructure, a first sheet of sinterized metal in contact with saidelectrode, and a second sheet of sinterized metal having anelectrocatalytic coating applied on one side thereof in contact with theface of the membrane without the electrode, the whole package insertedbetween two bipolar plates.

As the sinterized metal sheets are substantially rigid, the unavoidableloss of planarity of the bipolar plates may be only compensated bydeformations of the membrane, which is the weaker element from amechanical resistance standpoint. The membrane consequently is stronglystressed and may give rise to defects, in particular the presence ofgeometrical local irregularities, such as protruding peaks of thesinterized metal sheet and internal porosities of the membrane itself.This negative behaviour may be avoided only with a particularly accuratemechanical flattening of the surfaces of the bipolar plates. Further,the void ratio of the sinterized metal sheets is normally low andtherefore the flow of gases through said sheets involves high pressuredrops. As a consequence, the sinterized metal sheets may be used ascurrent distributors to substitute the meshes of U.S. Pat. No. 4,224,121but not as gas distributors. Therefore it is still necessary to usebipolar plates provided with grooves, with all the aforementionedproblems connected with the mechanical working and the relevant costs.

The problems above described affect also other types of electrochemicalcells, equipped with electrodes fed with hydrogen or oxygen, similar tothose used for fuel cells. Typical examples are electrochemical cellsfor the concentration of hydrogen or oxygen or for the electrolysis ofsalt solutions with gas depolarized electrodes.

BRIEF DESCRIPTION OF THE INVENTION

It is the main object of the present invention to provide for animproved electrochemical cell, such as a fuel cell, capable ofovercoming the problems and shortcomings of the prior art. Inparticular, the functions of transmission of electric current throughthe cell elements, the release of heat towards the external environment,the distribution of electric current to the electrodes and membranes,the removal of heat from the electrodes and membranes and thedistribution of the reactants and products are performed by distinctcomponents, in particular bipolar plates for the first two, porouselectroconductive collectors for the others. In view of this splittingof functions, the bipolar plates may have planar surfaces, withoutgrooves. Therefore, the electrochemical cells of the present inventioncomprise bipolar plates preferably made of aluminum, titanium or alloysthereof, obtained by cheap mass production techniques, such as cuttingfrom commercial sheets or casting in suitable molds. In particular, thebipolar plates do not require either mechanical flattening of thesurface or coating with an electroconductive protective film. Thebipolar plates of the present invention are used in combination withcollectors provided with deformability and residual resiliency andcapable of exerting high pressure in the areas of contact both with theelectrodes and with the bipolar plates. The collectors of the presentinvention are further characterized by high porosity and thereforeadvantageously act also as distributors for the reactants and products.In consideration of their high electrical and thermal conductivity, saidcollectors are capable of withdrawing heat from the membranes andelectrodes and efficiently transmitting it to the bipolar platesprovided with means for releasing the same. These and othercharacteristics of the present invention will be better illustrated inthe following detailed description and relevant examples, which are notto be considered in any case as a limitation thereof.

The present invention is particularly suitable for the construction ofimproved cell elements for electrochemical membrane cells, in particularfor low-temperature fuel cells, more particularly ion-exchange membranefuel cells. Said cells of the present invention are fed with reactantswhich may be gases containing hydrogen and oxygen, respectively to theanode compartment (negative polarity) and cathode compartment (positivepolarity) of each cell element and the products are both gases andliquids, such as water. As will be clear to the experts in the art, thepresent invention may be useful also for fields other than fuel cells,in particular for water electrolysis carried out directly on pure water,without electrolytes, also as steam, for the electrochemicalconcentration of hydrogen and oxygen from gasesous reactants containingthe same even in reduced percentages, for the production of oxygenperoxide by reduction of oxygen and for the electrolysis of varioussolutions with gas depolarized anodes or cathodes, when said processesare carried out in cells comprising cell elements having a structuresimilar to that of the cell element of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the appended drawings, wherein the same elements are indicated by thesame reference numeral. In particular:

FIG. 1 is a cross-section of a cell element of a cell according to thepresent invention:

FIGS. 2 and 3 are axonometric views of details of the cell elements ofthe present invention.

FIG. 4 is a cross-section of a gasket-frame coupled with an electrodeand a collector.

FIG. 5 is an axonometric view of a collector of the present invention.

FIG. 6 is a cross-section of an embodiment of the cell of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, the cell element of a cell of the inventioncomprises a pair of bipolar plates (1), a pair of collectors (14), apair of gasket-frames (8), a pair of electrocatalytic electrodes (7) andan ion exchange membrane (6).

With reference to FIG. 2, the bipolar plate (1) is made of a metal platewhich may have a flat surface in the area of contact with the collector(14). The peripheral frame area of the bipolar plate (1) is providedwith holes (2) and optionally with distribution channels (3) for theinlet and outlet of the gases, holes (4) for the passage of the tie-rods(not shown in the figure) and optionally internal ducts (5) for thepassage of a suitable cooling means. The dimensions of the bipolar plateare therefore dictated by the need to contain a certain active area ofmembrane (6) and electrodes (7) with the relevant collectors (14), aswell as the holes (2, 4) and channels (3). The main characteristic ofthe bipolar plates of the present invention is the possibility of beingproduced in a great number of pieces with reasonable costs by cutting ofcommercial sheets or by casting in suitable molds, without any furthermechanical flattening of the surface. The bipolar plates may be made ofaluminum, titanium or alloys thereof, without the need for anelectroconductive protective film. This last aspect will be illustratedin further detail in the following description. Obviously other metalsor alloys may be used, such as other valve metals (niobium, tantalum),stainless steels, also high-alloys steels, nickel-chromium alloys,although less cost-effective and heavier due to the higher specificdensity of these materials. When the construction material is aluminumor alloys thereof, the high thermal conductivity permits withdrawal ofthe heat produced during operation of the cell by cooling of theperipheral part of the bipolar plates only. For this reason theperipheral part is suitably enlarged and the heat removal may be carriedout by forced circulation of air or other cooling means (not shown inthe figures). According to this embodiment, the bipolar plates (1) madeof aluminum or alloys thereof are not to be provided with internal ducts(5), with much simpler construction and a substantial reduction of thecosts.

In FIG. 3 the gasket-frames (8) comprise holes (9) for inlet and outletof the reactants and products, fitting with the holes (2) of the bipolarplates (1), and optionally holes (10) for the passage of the tie-rods.The holes (10) are not necessary in a possible alternative embodimentwhere the corners are rounded off. The holes (9) are connected tosuitable channels (11) cut out in the thickness of the gasket-frame anddirected, coupled with channels (3), to uniformly distribute and collectthe reactants and the products inside the cell. Although not compulsory,preferably the products outlet should be localized in the lower part topermit an easier purging of the condensate water which may be formed inthe cell during operation.

The two faces of the gasket-frame may be non equivalent as, while theone in contact with the electrodes (7) and the membrane (6) may be flat,the one in contact with the bipolar plates is provided with channels(11) as aforementioned and with ribs (12), that is linear protrusionsdirected to ensure the necessary sealing to prevent gases from ventingoutside or mixing inside the cell. The sealing on the electrode side isensured by the intrinsic resiliency of each gasket-frame/membrane pair.For this reason the gasket-frame is made of an elastomeric castablematerial. The required resiliency must be sufficient to permit a safesealing under non-excessive mechanical load to avoid that deformationunder compression may obstruct channels (3) and (11) and that themembrane be excessively stressed in the peripheral area. The thicknessof the gasket-frame is dictated not only by mechanical considerationsbut also by the need to define an internal space available for thepassage of gas. The gasket-frame of FIGS. 3 and 4 is further providedwith a step (13) along the inside border to permit a ready housing ofthe electrode (7) and at the same time ensure a good protection of themembrane (6) from possible irregularities along the periphery of thecollectors (14), such as residual peaks or burrs from the cutting of thepieces having the desired dimensions from commercial sheets.

FIG. 4 shows in more detail the assembly made of gasket-frame(8)/collector (14)/electrode (7). The collectors (14) of the presentinvention are directed to provide at the same time for:

a multiplicity of contact points with the electrodes to minimize theenergy dispersions related to excessively long transversal paths ofelectric current inside the electrodes;

low values of the contact resistance with the surface of the bipolarplates preferably made of passivatable materials such as aluminum,titanium and alloys thereof, without protective electroconductive films;

heat transmission from the electrodes-membrane structures to the bipolarplates (1) optionally provided with ducts (5) where a cooling meansflows;

longitudinal flow of reactants with small pressure drop and uniformdistribution on the whole surface of the electrodes (7) due to the largepossibility of transversal mixing;

easy drain of liquid water formed by condensation inside the collectorduring operation;

deformability with sufficient residual resiliency under compression, asrequired to compensate unavoidable planarity defects of the variouscomponents of the cell, in particular the bipolar plates which ideallyare not subjected to precision mechanical finish of the surface. Acertain degree of residual resiliency is also necessary to maintainunder constant pressure the electrode/membrane structures to compensatethe thermal expansion of the various components during start-up, shutdown and electric load variations.

The above advantages are obtained by using collectors having a structuresimilar to a tridimensional network of metal wires, preferably fixed toeach other in interconnecting points. By suitably tailoring the diameterof the wires and the distance between interconnecting points, an optimumvoid ratio may be easily obtained which should preferably becharacterized by high values. The desirable dimensions of the voidsshould be sufficiently small in order to provide for the necessarymultiplicity of contact points, but also sufficiently large to minimizecapillary phenomena which would pose problems for the release of watercondensate. Said phenomena can be further reduced if the network ofmetal wires and the channels (3 and 11) are made hydrophobic, forexample by immersion in a solution containing a suitable hydrophobicagent followed by drying. A particularly preferred solution is anemulsion of polytetrafluoroethylene particles. Tridimensional networksof the above illustrated type are the mattresses described in U.S. Pat.No. 4,340,452 for use in electrolysis cells to ensure electricalcontinuity between a rigid current distributor and an electrode made ofa thin sheet, in the presence of an electrolyte having high conductivitywith moderate current density. Under these conditions optimum resultsare already obtained with modest pressures applied to the collector(tens-hundreds of grams per square centimeter) and with collectorsconsisting of a tridimensional network having relatively spaced apartinterconnecting points (some millimeters).

Preferably these mattresses are metal-wire fabrics or screens whereinthe wires form a series of coils, waves, or crimps or other undulatingcontours. More preferably the mattress consists of a series ofhelicoidal cylindrical spirals of wire whose coils are mutually woundwith the one of the adjacent spiral in an intermeshed or interloopedrelationship.

In the present case it has been found that for the best performance thevoids of the network must be such as to leave marks with dimensions inthe range of 0.1-3 mm on a pressure sensitive paper while the pressuresapplied to the bipolar plates are indicatively comprised between 0.1 and10 kg/cm². In an alternative solution, the tridimensional network may befurther characterized by a surface containing the terminal sections ofat least part of the metal wires: this feature compensates highlocalized pressures in very close limited-area points and therefore lowvalues of the contact resistance.

In FIG. 5 the collector (14) is represented by a network with a surfaceprovided with terminal sections (15), the efficiency of which has beendemonstrated by electric resistance measurements carried out onassemblies simulating a cell element of the cell of the inventioncomprising two planar plates in aluminum alloy obtained by castingwithout any further mechanical finish, two collectors made in nickel, 2mm thick, having a number of voids equal to 100/cm² (average dimensionof the voids: 1 mm), two electrodes commercialized under the trade nameof ELAT by E-TEK, U.S.A., holding inbetween a Nafion® 117 membranesupplied by Du Pont, U.S.A. The measured electrical resistances were inthe range of 100-5 milliohm/cm², with pressures of 0.1-80 kg/cm²respectively applied to the aluminum plates. The measured values arekept constant even maintaining the assembly in a steam atmosphere at100° C., as may happen during real operation.

Similar results have been obtained with metal plates made of titanium.The electrical resistances, measured under the same conditions with thesame assemblies, without the collectors of the present invention, showedvalues comprised between 200 and 1000 milliohm/cm², absolutelyunacceptable for a cell of industrial interest. The fact that theelectrical resistance is surprisingly low and stable with time also inthe presence of steam at 100° C. demonstrates that, contrary to what isknown in the art, the bipolar plates may be made of aluminum, titaniumor alloys thereof, without electroconductive protective coatings whenused in connection with the collectors of the present invention. As itis known that aluminum, titanium and alloys of the same are coated withtime by a film of an electrically insulating oxide, it may be assumed,without binding the present invention to any particular theory, that thehigh pressure localized in the limited-area contact points between thebipolar plates and the collectors of the present invention causes arupture of this film or prevents its growth. Said contact pressure isprobably about one order of magnitude higher than the pressure exertedon the bipolar plates.

Further, as already said, the bipolar plates may be advantageously usedas such after casting or cutting from industrial sheets, without anyneed of subsequent mechanical flattening. This result is probablyensured by the deformability of the collector and by its residualresiliency, which compensates for possible deviations from planarity,typical of unprocessed production pieces. As the deformability of thecollectors of the invention is relatively small at the pressure normallyapplied to the bipolar plates (in the order of some percent fractions ofthe thickness), it may be assumed that also the electrodes contribute tocompensating the planarity deviations of the bipolar plates. Inparticular, to maintain the stresses on the membranes within acceptablevalues, the electrodes must exhibit a significant deformability. Forthis reason, it has been found that the best results in terms of absenceof mechanical damage to the membranes are obtained when the electrodescomprise a deformable layer, such as carbon cloth. The bipolar platesmay be of both the grooved and flat type, the last one being preferredin view of the substantially lower manufacturing cost. As regards thestructure of the collector of FIG. 5, this tridimensional network may beobtained starting from an expanded foam having open cells in plasticmaterial, such as polyurethane, which is initially pre-treated to obtaina certain electrical conductivity (for example vacuum metallizing ormetal deposition by means of electroless baths as known in the art orpyrolysis under inert atmosphere or vacuum to form carbonaceousmaterial, optionally partially graphitized). The material, thuspretreated, is then subjected to galvanic deposition of the desiredmetal or alloy, for example nickel, copper or alloys thereof with othermetals, up to obtaining the desired thickness. The voids of the materialadvantageously have dimensions comprised between 0.1 and 3 mm and thediameter of the metal wires varies from 0.01 to 1 mm. Reference numeral(15) in FIG. 5 indicates the terminal sections of the metal wires which,as above illustrated, ensure for a multiplicity of contact points with ahigh localized pressure in the small areas represented by thecross-sections of such terminal sections. The thickness of thecollector, as will be clear from FIG. 1, is given by the thickness ofthe gasket-frame decreased by the thickness of the electrode. Thethickness of the collector is generally comprised between 0.5 and 5 mmand more preferably between 1 and 2 mm. The network of FIG. 5 isdescribed in EP publication 0266312.A1 which claims its use as anexpanded electrode for electrolysis of aqueous diluted solutions ofmetal ions and in U.S. Pat. No. 4,657,650 which describes itsapplication as the external electrical contact for the connection ofelementary cells in an electrolyzer.

Optionally, the tridimensional network (reticulated material) accordingto the present invention may also be used in connection with a metalmesh or graphitized carbon mesh interposed between the reticulatedmaterial and the electrode/membrane structure. In this double-layerstructure of the collector, the mesh, which may be particularly fine(for example meshes apertures smaller than 1 mm), ensures for thenecessary multiplicity of contact points with the electrodes, while thereticulated material may be more freely selected, for example withparticularly large voids in order to allow for the maximum percolationof the water which may have condensed inside. The use of the meshensures a higher protection of the membrane in the case the reticulatedmaterial presents a surface with particularly enhanced spikes.

In a further embodiment, the collector of the present invention issimply made of one or more superimposed meshes, made of woven metal wireindicatively having mesh apertures smaller than 3 mm, preferably smallerthan 1 mm, in order to ensure a multiplicity of contact points betweenthe electrodes and the bipolar plates. High contact pressures,particularly useful on the bipolar plate side, are obtained when thewire used for fabricating the meshes has a quadrangular cross-sectionbut also other polygonal cross-sections may be used. In this case thelongitudinal edges of the wire, in the superimposed points, form aparticularly useful array of asperities which imprint the metal surfaceof the bipolar plate. An alternative embodiment of the mesh which is aswell advantageous is represented by an expanded metal obtained bypre-cutting of thin sheets and subsequent expansion. In this way a meshis obtained with apertures having various forms, for example rhomboidal,the portions of the metal which define the mesh apertures being rotatedwith respect to the plane of the sheet itself. Therefore when theexpanded metal sheet is pressed against planar surfaces, the peaks ofsaid rotated portions of metal become the areas of contact. At least onepair of the above described meshes is used in order to provide forhigher resiliency and deformability, permeability to gaseous reactantsand percolation of the water condensate. For this last instance, themeshes may be characterized by different apertures, in particular a finemesh size for the one in contact with the electrodes and a coarser meshin contact with the bipolar plate.

A further embodiment of the present invention foresees the concurrentuse of the above described collectors of the invention and in particularthe reticulated material on one membrane side and one or more meshes,optionally having different mesh sizes, on the other side.

Further, the collector of the present invention made either ofreticulated material or of superimposed meshes may be used on one sideof the membrane only, while on the other side a rigid, conductive andporous material is used, such as a sinterized metal layer. This must besufficiently thin in order to comply with the profile of the bipolarplate, which is not perfectly planar, under the applied pressure. Thevoid ratio and the dimensions of the pores of the sinterized metal layermust be of the type already described for the collectors of theinvention in order to permit the flow of reactants and products,percolation of the water condensate and multiplicity of contact pointswith the electrodes and bipolar plates. The metal forming the collectorof the present invention must resist possible aggressive conditionswhich may be particularly severe when the cell is fed with air on thepositive pole compartments and/or with a mixture of carbon dioxide andhydrogen on the negative pole compartments. Under these conditionspossible water condensates are acidic. Taking into account both thispossibility and the fact that the operating temperature is higher thanthe room temperature, most advantageously the metal is stainless steelof the 18 chromium-10 nickel type, preferably high-alloy steel,nickel-chromium alloys, titanium, niobium, or other valve metals. Thecollectors and the bipolar plates of the invention may be optionallycoated with an electroconductive protective film, for example made ofplatinum group metals or oxides thereof. Alternatively the protectivefilm may be made of conductive polymers of the type comprisingintrinsically conductive materials such as polyacetylenes, polypyrroles,polyanilines or the like or plastic materials containing conductivepowders (for example graphite powder).

FIGS. 1 and 6 clearly illustrate that each pair of bipolar plates (1) inaluminum or other passivatable material or alloys thereof maintainspressed inside a pair of collectors (14) of the invention, a pair ofelectrodes (7) and a membrane (6). Said electrodes, as known in the art,before insertion between the bipolar plates and the collectors, arebonded to the membrane under pressure and heating, possibly afterapplying to the surface of the electrodes a suspension or solutioncontaining the polymer forming the membrane, with the aim to facilitateboth the adhesion of the electrode to the membrane as well as theformation of a large area of triple contact between gas, membrane andcatalytic particles of the electrodes. If the membranes and theelectrodes are bonded together to form a unitary structure, the bipolarplates and the collector of the invention however do not produce anyappreciable improvement of the cell performance with respect to theteachings of the prior art. Therefore, the advantages of the presentinvention in this case are limited to higher simplicity and lowerproduction costs, in particular for the bipolar plates made of aluminumor other passivatable metals without any protective coating.

It has been surprisingly found that the bipolar plates and thecollectors of the present invention obtain optimum cell performancesalso when the electrodes, contrary to what is taught in the art, are notpreviously bonded to the membrane, which obviously reduce the productioncosts and to limit the risk of damaging the delicate membranes. Withoutbinding the validity of the present invention to any particular theory,it may be assumed that the multiplicity of contact points and the highpressure obtained on said points, typical of the above mentionedcollectors, are capable of maintaining a high percentage of the area ofthe electrodes in intimate mechanical contact with the membrane. As aconsequence, the number of catalytic particles embedded in the membranesurface (triple contact area) is analogous in the case of the presentinvention with the electrodes only laying onto the membrane and in thecase described in the prior art with the electrodes bonded to themembrane. Conversely, it has been found that with collectors consistingof undulated sheets or simply grooved bipolar plates, as known in theart, the performances are acceptable only when the electrodes are bondedto the membrane. As above discussed, with these collectors the contactareas under sufficiently high pressure are limited only to the crossingpoints of the grooves or undulations and therefore involve a limitedportion of the electrodes surface, which is the only one to be kept incontact with the membrane. In the remaining portion of electrodesurface, the contact pressure with the membrane is nil and duringoperation the differential expansion of the membrane and the electrodesmay bring about separation of the surfaces. This remaining portion,therefore, does not contribute in any way to the performance of thecell. These considerations are to explain why the prior art describesbonding of the electrodes to the membrane as an essential factor for agood performance of the cells with collectors provided with grooves orundulations.

The optimum results obtained according to the present invention withelectrodes not bonded to the membrane are probably due also to a secondcharacteristic of the collectors, that is deformability and residualresiliency under compression. This characteristic compensates for thesmall deviations from planarity of the planar bipolar plates notsubjected to mechanical flattening of the surface.

The compensation of planarity defects maintains a uniformly distributedcontact over all the surface of the bipolar plates, of the electrodesand membranes, ensuring thus optimum performances by a homogenousdistribution of current. As already said, in order to maximize thedeformability property, advantageously the electrodes (7) may have adeformable structure. Therefore, even if the electrodes may be made astaught in the art, in the form of porous sheets obtained bysinterization of a mixture comprising powders of electroconductive andelectrocatalytic materials, a polymeric binder and optionally agentssuitable for favouring the formation of pores, advantageously they aremade of a porous deformable layer of conductive material. Onto saidlayer, a suspension is applied by spraying or brushing or any similartechnique. The suspension comprises a liquid vehicle, powders ofelectrocatalytic and electroconductive materials and polymeric binder,optionally comprising ionic groups, with hydrophobic or hydrophiliccharacteristics, directed to control the wettability of the system. Theporous layer is then dried and subjected to a thermal treatment directedto mechanically stabilize the applied material. Suitable layers are madeof carbon cloth or paper, optionally graphitized. The carbon cloth ispreferred in view of the higher deformability and flexibility whichfacilitates handling and assembling into the cell. Products of thistype, containing platinum as the catalyst and polytetrafluoroethylene asthe polymeric component are commercialized by various companies, forexample E-TEK, U.S.A. under the trade name of ELAT. These products maybe utilized as such or after painting with a suspension or paintcontaining a ionic polymer similar to that forming the membrane. Furthertypes of porous layers are made of metal sinterized layers or finescreens or multilayer clothes, for example made of various types ofstainless steels, high-alloyed steels, or alloys of nickel, chromium andtitanium. Generally multilayer cloths are best preferred in view oftheir deformability. In another embodiment, the above layers, when madeof a multi-layered cloth, may act at the same time as collectors andelectrodes. In this case, the aforementioned suspension containing theelectrocatalytic particles is applied only to the surface to be put incontact with the membrane.

FIG. 6 describes the assembly made of a multiplicity of cell elements ofFIG. 1 to form the cell of the invention, comprising the bipolar plates(1), collectors (14), electrodes (7), gasket-frames (8), ion exchangemembranes (6), end-plates (18), pressure plates (17). The bipolar plates(1) are provided with external connections (16) which, once connected,short-circuit two or more bipolar plates of the cell elements in thecase of malfunctioning. The same result could be obtained with bipolarplates provided with recesses of suitable form. This type of actionpermits the safe operation of the cell comprising a high number of cellelements connected in electrical series and it is therefore extremelyhelpful from a practical point of view. It must be further noted thatshortcircuiting is efficient only if the ohmic drop in theshortcircuited bipolar plates transversally crossed by electric current,is negligible: this is obtained with bipolar plates made of highlyconductive materials such as aluminum or alloys thereof.

The following examples, which do not constitute in any case a limitationof the objects of the present invention, will better explain the presentinvention. For convenience sake the examples have been limited to thecase of the fuel cells.

EXAMPLE 1

4 fuel cells, each one made of three cell elements comprising twopressure plates (17 in FIG. 6), two end-plates (18 in FIG. 6) and twobipolar plates (1), three pair of collectors (14), three pairs ofelectrodes (7), three membranes and three pairs of gasket-frames (8),were assembled as illustrated in FIG. 6. The general operatingconditions, kept constant during all the tests, were as follows:

dimensions of the electrodes and collectors: 10×10 cm²

membranes: Nafion® 117, supplied by Du Pont, U.S.A.

membrane active area: 10×10 cm²

moulded gasket-frames, having inside dimensions of 10×10 cm² and outsidedimensions of 20×20 cm², a thickness of 2 mm, provided with holes (9)and (10), channels (11), ribs (12) 0,1 mm high, internal step (13) 0.5mm deep with an external dimension of 11×11 cm², as shown in FIG. 3.Construction material: Hytrel® commercialized by Du Pont, U.S.A.;

bipolar plates and end plates with external dimensions of 20×20 cm²,provided with holes (2) and (4) and other characteristics as specifiedhereunder.

feed to the anode (negative) compartments made of pure hydrogen at 2atm, pre-heated and pre-humidified at 70° C. in an external saturator,the flow rate being doubled with respect to the stoichiometry of thereaction;

feed to the cathode (positive) compartments made of purified air at a2.1 atm, pre-heated and pre-humidified at 50° C. in an externalsaturator with a triple flow rate with respect to the stoichiometry ofthe reaction;

operating temperature: 80° C.

total current: 50 Ampere, corresponding to a current density on theactive area of the electrodes equal to 5000 Ampere/m2

total operation time for each test as indicated here after, but in anycase comprised between 300 and 400 hours, with start up and shut down atthe beginning and at the end of each working day.

Each fuel cell was equipped with a combination of the followingalternatives:

A. bipolar plates and end plates in aluminum alloy, UNI 5076 type(Italian Standards), obtained by pressure die-casting having a thicknessof 5 mm, provided with internal ducts for cooling (5) made of stainlesssteel of the 18 chromium-10 nickel type with an internal diameter of 3mm and channels (3) as illustrated in FIG. 1 and 2

B. same bipolar plates and end plates as in A, the only difference beingthe construction material, titanium instead of aluminum alloys.

C. bipolar plates and end plates made of aluminum alloy, Anticorodal 100TA16 type (Italian Standards) obtained by cutting from commercial sheetshaving a thickness of 3 mm, without inside cooling ducts (5) andchannels (3). In this case the external dimension of the plates wasincreased to 30×30 cm² to permit cooling by forced air.

D. same bipolar plates as in C, but with the contact surface with thecollectors coated with a chromium film obtained by galvanic deposition;

E. same bipolar plates as in C, but with the contact surface with thecollectors coated with a film of a polymeric conductive film belongingto the group of polyaniline.

F. electrodes made of flexible conductive carbon cloth, coated on oneside thereof with a film containing electrocatalytic platinum particlessupported onto active carbon and a polymeric binder and on the otherside with a hydrophobic porous and conductive film based onpolytetrafluoroethylene, supplied by E-TEK, U.S.A. under the trade nameof ELAT, 0.5 mm thick, with a platinum load of 0.5 mg/cm² ;

G. same electrodes as in F, with the further application of a polymersimilar to that of the membrane on the side containing the catalyst,applied by brushing or spraying with a solution of a perfluorinatedpolymer containing sulphonic groups, commercialized by SolutionTechnology, U.S.A. under the trade name of Nafion Solution 5%

H. same electrodes as in G, wherein the flexible carbon cloth issubstituted for a conductive carbon rigid graphite paper supplied byToray, Japan, under the commercial trade name of TGHP 030;

I. same electrodes as in G, wherein the flexible carbon cloth issubstituted for a multilayer cloth in stainless steel of the type 18chromium-10 nickel-2 molybdenum;

L. collectors in reticulated material as shown in FIG. 5, made of analloy of 50 chromium-50 nickel, having an average diameter of the poresof about 0.2 mm and a 2 mm thickness. Materials of this type arecurrently supplied by different companies and are conventionally calledmetal foam;

M. same collectors as in L, having an average diameter of the pores ofabout 1 mm

N. same collectors as in L, having an average diameter of the pores ofabout 3 mm;

O. collectors consisting of 3 superimposed meshes made of 18 chromium-10nickel stainless steel wire having a diameter of 0.3 mm forming mesheswith apertures of 0.5×0.5 mm;

P. collectors consisting of 2 titanium expanded sheets having diamondshaped apertures with the major dimension respectively of 1 mm (expandedsheet on the electrode side) and 3 mm (expanded sheet on the bipolarplate side) obtained from a sheet 0.5 mm thick and coated with aplatinum layer 0.3 microns thick formed by galvanic deposition;

Q. collectors consisting in a multilayer cloth obtained from a metalwire having a diameter of 0.15 mm made in stainless steel of the 18chromium-10 nickel-2 molybdenum type, 2 mm thick under compression,commercialized by Costacurta, Italy;

R. collectors made of a layer of sinterized metal such as stainlesssteel of the type 18 chromium-10 nickel, 2 mm thick.

The average voltages referred to the cell element and expressed asVolts, are shown in Table 1, for fuel cells equipped with bipolar platesof the type A.

The temperature of the plates was controlled by forced circulation ofdemineralized water at 75° C.

                  TABLE 1                                                         ______________________________________                                        electrode    F      G          H    I(**)                                     ______________________________________                                        collector                                                                     L(*)         0.65   0.7        0.7  ==                                        L            0.6    0.7        0.6  ==                                        M            0.6    0.7        0.6  ==                                        N            0.55   0.65       0.6  ==                                        O            0.6    0.7        0.65 ==                                        P            0.6    0.7        0.6  ==                                        Q            0.6    0.7        0.65 0.7                                       R            0.45   0.5        0.45 ==                                        L + P(***)   0.6    0.7        0.6  ==                                        L + R(***)   0.6    0.65       0.6  ==                                        ______________________________________                                         (*)data obtained with electrodes bonded to the membrane                       (**)the multilayer cloth acts as electrode and collector at the same time     (***)collectors made of sinterized materials of the type R and                superimposed expanded meshes of the type P installed in the anode             (negative) compartments.                                                 

The data reported in Table 1 may be commented as follows:

the data obtained with electrodes bonded to the membrane (line L*)represent a comparison with the prior art. It is clear thatpre-treatment of the electrodes with a polymer solution similar to thatof the membrane allows for a definite improvement of the performances.

the multiplicity of contact points per unity of surface area isinstrumental for an optimum performance. The tridimensional network ofthe type N, characterized by pores with an average dimension of 3 mm, isin fact constantly characterized by insufficient voltages.

the deformability of the collectors and electrodes is a key factor asdemonstrated by the unsatisfactory voltages obtained with the sinterizedmaterials (line R) and the rigid graphite paper used as a substrate forthe electrodes (column H).

in the case of collectors made of sinterized material (line R), theunsatisfactory performances are also due at least to a partial floodingof the compartments (probably the cathode positive compartments) due tothe water condensate formed during operation and retained by capillarityin the small pores of the sinterized material;

the optimum and stable voltage values, typical of all the tests,demonstrate that the electrical resistance between the collectors of theinvention and the planar bipolar plates of aluminum alloys, without anelectroconductive protective coating, is extremely reduced. This resultis quite surprising in consideration of the fact that aluminum andalloys thereof are known to become coated with a natural electricallyinsulating oxide, in particular under heat in the presence of steam(typical operating condition of the fuel cell). A confirmation of thisconclusion is given by the voltages quite similar to those obtained withbipolar plates coated with a chromium protective film (type D) andconductive polymeric material (type E).

EXAMPLE 2

The same test of Example 1, characterized by utilizing electrodes of thetype G and collectors of the type R (sinterized material) was repeatedafter having made the bipolar plates, end plates and collectorshydrophobic by immersion in a suspension of polytetrafluoroethylene(commercialized by Du Pont under the trade name of Teflon 30N) followedby thermal treatment at 150° C. The voltages measured under the sametesting conditions of Example 1 resulted comprised between 0.55 and 0.65Volts. This improvement may be ascribed to the lower tendency of thesinterized material to retain water formed by condensation duringoperation.

EXAMPLE 3

The same cell of Example 1, characterized by the the presence ofelectrodes of the type G and collectors of the type L was subjected torepeated shortcircuiting of the second cell element by connecting bymeans of clamps the external connections indicated by reference numeral16 in FIG. 1 The average voltages of the other cell elements did notchange during the shortcircuiting periods and the shortcircuited cellelement shortly reached the normal voltage once the clamps weredisconnected. The maximum voltage between the bipolar plates of theshortcircuited cell element during shortcircuiting resulted in the rangeof 20-30 mV.

EXAMPLE 4

The influence on voltage of the different type of bipolar plates and endplates were examined by repeating the test of Example 1 using theelectrodes of type G and the collectors of type L and substituting thecast bipolar plates and end plates in aluminum alloy (type A) forsimilar ones made of titanium (type B). Average voltages of the unitaryelements comprised between 0.68 and 0.71 Volts were detected,substantially similar to those typical of the fuel cell equipped withbipolar and end plates in aluminum alloy. Similar results were obtainedby further substituting the bipolar plates of the type B with plates inaluminum alloy of the type C. Cooling was carried out by forcedcirculation of pre-cooled air fed through separated ducts located beloweach cell element.

EXAMPLE 5

A series of test was carried out to obtain further comparative data withthe prior art. Two fuel cells were made of three cell elementscomprising bipolar plates and end plates provided with grooves directedto act as current distributors and consisting respectively of graphiteand aluminum alloy of the type UNI 5076. The bipolar plates and terminalplates were further provided with internal ducts for cooling.

The grooves were oriented in order to be crossed at 90° for each pair offacing sides of the bipolar and end plates.

The electrodes were of the type G of Example 1 and the membranes were ofthe type Nafion® 117. The fuel cell equipped with bipolar plates andterminal plates in graphite and with the electrodes bonded to themembrane was operated under the same conditions as in Example 1 andresulted characterized by the best average voltages referred to the cellelements measured in the various conditions reported in Table 1 (0.7Volts). However, the same fuel cell, provided with electrodes of thetype G not bonded to the membrane, showed quite unsatisfactory averagevoltages, comprised between 0.5 and 0.55 Volts, thus demonstrating thatonly the collectors of the invention, with their multiplicity of contactpoints, are capable of ensuring a satisfactory and extended continuitybetween the surfaces of the membranes and those of the electrodes, whenthese are not previously bonded.

As said above, the fuel cell, comprising grooved bipolar and end platesin aluminum alloy and electrodes of the type G bonded to the membrane,showed quite satisfactory performances at the beginning of the test.However, the voltages rapidly decreased to low values (0.4 Volts) inabout a hundred hours, thus demonstrating that only the collectors ofthe invention are capable of maintaining the contact resistance withinnegligible values with time.

As a confirmation of this fact a further test was carried out with afuel cell comprising grooved bipolar and end plates in aluminum alloy,electrodes of the type G (Example 1) not bonded to the membranes andcollectors of the invention of the type M (Example 1). The voltageresulted satisfactory (0,60 e 0,65 Volts) and stable with time. Further,this embodiment resulted particularly efficient for draining the watercondensate formed in the cathode (positive) compartments where thegrooves of the bipolar and end plates were positioned in the verticaldirection.

Various modifications of the cell of the invention may be made withoutdeparting from the spirit or scope thereof and it has to be understoodthat the present invention is limited only as defined in the appendedclaims.

We claim:
 1. A fuel cell comprising pressure plates (17), bipolar plates(1) or end plates (18) made of metals, or metal alloys, provided withholes (2) for feeding the gaseous reactants and removing the productsand the residual reactants,current collectors (14) permeable to gasflow, electrocatalytic porous electrodes (7) bonded to ion exchangemembranes (6) before assembling of the fuel cell and gasket-frames (8)characterized in that said collectors (14) consist of a porouseletroconductive material having residual deformability and resiliencyunder compression and are provided with a multiplicity of limited areacontact points said bipolar plates and the end plates are made of metalsor metal alloys capable of forming a protective oxide.
 2. The fuel cellof claim 1 wherein said bipolar or end plates (1,18) have an enlargedperipheral part for heat removal by cooling means.
 3. The fuel cell ofclaim 1 wherein said collectors (14) provided with residualdeformability and resiliency are a tridimensional network of metalwires, the surfaces of said network containing the terminal sections(15) of at least part of said wires.
 4. The fuel cell of claim 3 whereinsaid tridimensional network has a porosity at least equal to 50% and adiameter of the metal wires comprised between 0.01 and 1 mm.
 5. The fuelcell of claim 1 wherein said collectors (14) provided with residualdeformability and resiliency consist in at least two superimposed meshesmade of metal wires having polygonal cross-section.
 6. The fuel cell ofclaim 5 wherein said meshes have a different mesh size with a finer meshfor the mesh in contact with the electrodes (7) and a coarser mesh forthe mesh in contact with the bipolar or end plates (1,18).
 7. The fuelcell of claim 1 wherein said collectors (14) have voids with dimensionscomprised between 0.1 to 3 mm.
 8. The fuel cell of claim 1 wherein saidcollectors (14) have a thickness comprised between 0.5 and
 5. 9. Thefuel cell of claim 1 wherein said collectors (14) are made ofcorrosion-resistant material selected from the group comprisingstainless steels, high-alloy steels and nickel-chromium alloys.
 10. Thefuel cell of claim 1 wherein said collectors (14) and said bipolar orend plates (1,18) are made hydrophobic.
 11. The fuel cell of claim 1wherein said bipolar or end plates (1,18) have a planar surface.
 12. Thefuel cell of claim 1 wherein said bipolar or end plates (1,18) areobtained by molding or cutting of commercial sheets without furtherfinish of the surface.
 13. The fuel cell of claim 1 wherein said bipolaror end plates (1,18) are further provided with channels (3) for thedistribution and for the removal of the reactants and products.
 14. Thefuel cell of claim 1 wherein said bipolar or end plates (1,18) arefurther provided with internal ducts (5) for cooling with a gaseous orliquid means.
 15. The fuel cell of claim 1 wherein said metals or metalalloys capable of forming a protective oxide are selected from the groupcomprising aluminum, titanium, zirconium, niobium, tantalum and alloysthereof and stainless steels.
 16. The fuel cell of claim 15 wherein thecontact resistance between said bipolar or end plates (1,18) and saidcollectors (14) is comprised between 100 and 5 milliohm/cm² with apressure exerted on said plates (1,18) comprised between 0.1 and 80kg/cm².
 17. The fuel cell of claim 1 wherein said gasket-frames (8) aremade of castable elastomeric material and comprise holes (9) for thedistribution and the removal of the reactants and products and a step(13) for the housing of said electrodes (7) and ribs (12) for sealingand separation of the reactants and products.
 18. The fuel cell of claim1 wherein said electrodes (7) are made of a porous, conductive layerprovided with a surface containing a catalyst and a surface containinghydrophobic material.
 19. The fuel cell of claim 18 wherein said layeris a flexible carbon cloth.
 20. The fuel cell of claim 18 wherein saidlayer is a carbon paper.
 21. The fuel cell of claim 18 wherein saidlayer is a flexible cloth of corrosion resistant metal selected from thegroup comprising stainless steels, high-alloy steels and nickel-chromiumalloys.
 22. The fuel cell of claim 18 wherein said electrodes (7) arefurther provided with a coating of a polymer having ion exchangecharacteristics applied onto the surface containing the catalyst. 23.The fuel cell of claim 1 wherein said bipolar or end plates (1,18) areprovided with external connections or recesses (16) suitable forshort-circuiting.